The disclosure relates generally to manipulation of fluid flows using an active flow control system and more particularly, a method for using an active flow control system to achieve both lift enhancement and destruction.
An airfoil-shaped body moved through a fluid produces a force perpendicular to the motion called lift. Subsonic flight airfoils have a characteristic shape with a rounded leading edge, followed by a sharp trailing edge, often with asymmetric camber. A fixed-wing aircraft's wings, horizontal, and vertical stabilizers are built with airfoil-shaped cross sections, as are helicopter rotor blades. Airfoils are also found in propellers, fans, compressors and turbines. Of concern is active circulation control of these aerodynamic structures. More specifically, of concern is the utilization of active circulation control for aerodynamic structures, such as a wind turbine blade or a gas turbine blade, to achieve both lift enhancement and destruction dependent upon need, as compared to the same blade without active circulation control.
Airfoil circulation control typically uses fluid injection in the form of a secondary fluid flow to create a steady wall-jet at the proximity of a rounded surface in a blade to leverage the Coandã effect. The Coandã effect can be defined as the effect by which a fluid jet attaches itself to an adjacent surface, such as an airfoil, and remains attached. Circulation control may result in increased lift and systems using this principle have been conceptualized for a wide variety of applications from aircraft wings to wind turbines. In aircraft wings applications, the circulation control may work by increasing the velocity of the airflow over the leading edge and trailing edge of a specially designed aircraft wing using a series of blowing slots that eject high pressure jet air tangentially as the secondary fluid flow, in a substantially downstream direction as relates to the incoming primary fluid flow. The wing has a rounded trailing edge to tangentially eject the air through the Coandã effect, thus causing lift. The increase in velocity of the airflow over the wing may also add to the lift force through conventional airfoil lift production. In other systems, the injection of the secondary fluid flow creates or enhances separation over the aerodynamic surface for lift destruction by creating a flow disturbance on or near the aerodynamic surface. As described, a method that can accomplish both lift destruction and lift enhancement in a single active system does not exist.
Since their conception, airfoils have suffered the risk of stall, or loss of lift, due to flow separation over the surface. In particular, it is known that airfoils at high angles of attack are at risk of the incoming primary flow separating from the surface of the airfoil, causing loss of lift. In addition, newer airfoils used for energy capture may suffer damage due to increased lift in unexpected high flow conditions. Furthermore, blade-to-tower clearances in wind turbines are of concern as a result of aerodynamic loading on the blades causing them to bend toward the tower. Stiffer, and thus more expensive, blades may be required to avoid collision with the tower. By reducing the aerodynamic load on the blade as it is passing in front of the tower, the risk of blade-to-tower collision would be minimized, if not eliminated. In addition, improving lift when the blade is not passing in front of the tower may provide increased energy production.
It is therefore desirable to achieve circulation control around an aerodynamic structure, such as in airfoils at high angles of attack, to provide lift destruction in unexpected high flow conditions, as a result of flow blockage over the surface of the airfoil, or alternatively increase lift producing capability, as a result of flow separation over the surface of the airfoil to minimize the risk of stall. It is additionally desirable to provide such a system configured to provide both lift enhancement and destruction, dependent upon need and current conditions, but to do so at a reduced system cost. More specifically, it is desirable to provide a blade, such as for using in wind turbines, turbomachinery, aerospace vehicles, and the like, that is optimized or designed to provide a single active system that provides better load-bearing performance than other currently commercially available streamlined aerodynamic profiles. Therefore, there is a need for an improved airfoil active flow control method that address one or more of the aforementioned issues.